The unpredictability of scyphozoan jellyfish blooms

Abstract

The study of jellyfish blooms has gained attention in the recent decades because of the importance of forecasting and anticipating them and avoiding their interference with human activities. However, after thirty years of scientific effort (monitoring systems, empirical laboratory and field studies, modeling, etc.), the occurrence of blooms remains unpredictable, and their consequences unavoidable. Climate change, eutrophication, overfishing, coastal construction, and species translocation have been suggested as stressors that increase them, but robust evidence to support these claims is limited. The widespread belief that jellyfish blooms are “increasing in number” has been challenged in recent years. Among the gelatinous zooplankton, the bloom forming species are concentrated in the class Scyphozoa, and the number of species with at least one recorded bloom has increased during the last decade. The analyses of long-term time series show seasonality in the dynamic of each blooming jellyfish species population, but the blooms vary in intensity and there are years of an unexplained absence of jellyfish. In this review, we focus on the current state of knowledge, uncertainties and gaps in the critical points that can strongly influence the intensity of the bloom or even lead to the absence of the medusa population. These points include ephyrae, planulae and scyphistoma natural, predatory or fishing mortality, the molecular pathway of strobilation, benthic population dynamics, planula settlement and ephyra to medusa transition success. Some of these points account for certain empirical laboratory evidence under controlled conditions, and are difficult to be studied on the field, but the different sources of non-typically recorded variability need to be addressed to improve our understanding of jellyfish population dynamics.

Coupling hydrodynamic drifting simulations and seasonal demographics to forecast the occurrence of jellyfish blooms

“…Empirical records as to how the transition of jellyfish individuals through their complex and cryptic lifecycles influences the spatial dynamics of their populations do not currently exist at the resolution necessary for quantitatively predicting the exact timing and location of jellyfish bloom occurrences (Fernández-Alías et al, 2024). This lack of capacity is largely because identifying jellyfish polyp beds is imperative for understanding the subsequent movements of medusae aggregations, and yet these seed locations remain frustratingly mysterious (Gibbons & Richardson, 2013).…”

“…Moreover, while it is debated whether the causal mechanisms are increasing global jellyfish abundances or increased human exploitation of coastal environments, there is a growing perception that the recurrence frequency of jellyfish bloom events is increasing (Condon et al, 2013). Regardless of this apparent increase, however, the dynamics of jellyfish populations and thus the causal mechanisms of bloom formation remain poorly understood (Fernández-Alías et al, 2024; Goldstein & Steiner, 2020; Kennerley et al, 2021), leaving us unable to mitigate their broad ecological and economic impacts (Condon et al, 2013).…”